Harald:

It's time for the evening frolics to start here.
Compression and tension, given reasonable strength, never cause a rod failure.

There are two sources of bending load, bearing friction and distortion of the crankcase and crankshaft.

Bearing friction is obvious, so long as the rod shank is wide enough to withstand this load no further consideration is needed.

In a well designed crankcase thermal distortion is kept to a minimum, the crankshaft stays at a 90 degree angle to the cylinder.

The load of the piston and crankpin, pushing against each other, caused the crank to flex. this is load is greatest when the cylinder fires, and is the time of greatest flex. This flexure is minimized by having bearings on both sides of the crankshaft throw. In a model engine this is seldom used because of assembly considerations and weight.

This flex causes the big end bearing of the conrod to have much higher bearing load on the edge next to the web of the shaft, and if too high can wipe the oil film out, then the bearing runs dry and fails, leading to a broken rod. This failure is almost never seen in a model engine.

This constant flexing is the cause of most crankshaft failures, however.

At the same time this off-center load on the bearing will cause the conrod also, to flex. This happens constantly while the engine is running, and can cause a fatigue failure.

All this is supposing the engine is in good condition, with correct clearances in the big end bearing. Extended running at elevated rpm makes the bearing loosen, bringing another set of problems.

With a loose big end, centrifugal force tends to throw the big end outwards, then when the cylinder fires the big end is hammered down on the crankpin. This can cause the metal of the conrod to crystallize, leaading to a fatigue fracture. This is a common failure in the engines used in model racing cars. It also aggravates the tendency to break the crankshaft.

Do not "Airfoil" the rod, this will lighten it, but it weakens it at the same time. Drilling holes will lighten the rod, but unless carefully polished after drilling scratches are left, these scratches are places for fatigue cracks to start.

All this leads to my way of lightening a conrod, which I (obvoiusly) think is best, the greatest lightening while retaining its strength.

Leave the edges approximately square, and polish to remove all tool/filing marks.

Then, leaving lands about 2/3 the width of the edges, cut a groove in each side leaving the metal in the center at 10% to 15% of its original thickness. Be very careful to polish all marks out of these grooves, again, a scratch is a focus for the start of a crack.

You have now made the rod into an "H" section, which except for a tubular rod, is the strongest possible, and the lightest. It is also the hardest to do well. I have never broken a rod done this way, but I have broken crankshafts.

Yes, exotic metals, such as titanium, can be used, but I don't think there is any gain in their use. And they can cost a lot more.

Hope this answers more questions than it raises.

Alloy rods do fine, for titanium get a gold mine.

Bill.